PO voucher going thru doc tolerance check app engine (FS_DOC_TOL) gets stuck with IN_PROCESS_FLG = '

We are on Peoplesoft Financials 9.2, PUM 20, it happens occasionally where a voucher among a couple of hundred vouchers will go thru the doc tolerance check app engine (FS_DOC_TOL) batch process gets stuck with IN_PROCESS_FLG = 'Y'. We cannot identify a pattern, it seems random and the app engine process completes successully with no errors. I've also turned on tracing and reviewed the trace and don't see anything that stands out. When we reset that IN_PROCESS_FLG of the affected voucher thru a SQL update, then run it thru the batch process again by itself, it works fine. Has anyone